Stanton D. answered • 02/28/22
Tutor to Pique Your Sciences Interest
Hi 0000 A.
Another clueless tutor ....
So you need a physical characteristic that you can definitely sort these spidery bodies (let us say) into 2 distinct groups by. (If they were spiders, you would group by number of eyes. But they aren't.)
Not so easy once you take all the legs off, is it!
Some appear to have darkly pigmented spots on their antennae, which MIGHT be eyes. That would require closer inspection, and would definitely be distinctive.
Some appear to have greatly enlarged abdomens vs. the thorax. However, segmentation is not apparent, so a fused abdominothorax would not necessarily afford a wise choice (variation might require an arbitrary division criterion).
Some appear to carry appendages forward, and others, lateral, from their thorax, and the appendages may be variously jointed -- two appear to be fin-like, two divided with claws, ahd the rest simply hooked terminally. There are several ways those could be dichotomously grouped.
General pigmentation patterns aren't well characterisable for dichotomous-key purposes, although they may be distinctive for certain species.
You might want to look at some insect and other macroinvertaebrate charts, to get some background familiarity with the population, so to speak. Though, I suspect these are "artificial" forms, just to encourage YOU to grind your mental gears along the lines above. (And why didn't you? Or is this question just a check for your answer? Eh, "Asked"?)
--Cheers, --Mr. d.
